Foaming & Carry-Over in Oil/Gas Separators: A Study

Gas Sep. Purif:
Vol. 9, No. 2, pp. 81-86, 1995
0 1995ElsevierScienceLtd
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A study of foaming and carry-over
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problems
in oil and gas separators
Habib.
Department
I. Shaban
of Chemical
Engineering,
Kuwait
University,
PO Box 5969,
13060
Safat,
Kuwait
In oil fields, separators
are used to separate
oil and gas contained
in the crude oil pumped
from the wells
before
processing.
Although
there
are many
factors
influencing
the
performance
of these separators,
one of the crucial
problems
is the formation
of foam due
to the impurities
present in the crude. Another
operating
problem
is carry-over,
which occurs
when free liquid escapes with the gas phase. Of the several methods
used to control
foam,
chemical
control
by use of antifoam
agents
is very important.
In this work, the main
objective
is to study the poor separation
of oil and gas due to foaming
and the carry-over
problem
in separators
observed
in one of the oil fields
in Kuwait.
Studies
were also
conducted
on the effect of a silicone
antifoam
agent used to control
foaming
in order to
increase the separation
efficiency
and thereby
increase the production
capacity.
Keywords:
oil and
gas
separators;
foaming;
Introduction
The oil and gas composing well fluid are usually
separated in separators. A separator is essentially a vessel
whose interior is kept at a prescribed separation pressure
and temperature. The three principles used to achieve
physical separation are momentum, gravity settling and
coalescing. Any separator may employ one or more of
these principles, but the three phases must be immiscible
and have different densities for separation to occur. Some
oil and gas separations are simple while others are very
complex’.
Separators may be vertical cylindrical, monotube and
dual-tube horizontal, or spherical, depending on their
design’. There are several factors which affect the
performance of a vapour-liquid separator. The temperature and pressure are two important factors which affect
the separation for a feed of given composition. With
increasing temperature and/or decreasing pressure, the
volume of vapour increases and the volume of liquid
decreases. In addition to this, there is a decrease in the
densities of both the vapour and liquid phase. Compared
with the other changes involved, this density change is
more significant ‘.
Some systems may contain small amounts of
surfactants, which cause the formation
of foam in
vapour-liquid
systems and emulsions in liquid-liquid
systems. The major cause of foam is impurities (other
than water) in the crude oil which it is not practical
to remove before the stream reaches the separator. The
formation of foam and emulsions adversely affects the
carry-over;
antifoam
agents
performance of separators, especially when they are
stable. The presence of surfactants is usually not known
in advance, so separators are designed presuming that
they are not present. Foam creates no problem within
the separator if the internal design ensures adequate time
or a sufficient coalescing surface for the foam to break.
In addition to this, the size of the entrained particles is
one of the important properties affecting separation
efficiency. Other factors influencing separation are gas
velocity, surface tension and viscosity3.
Foaming in the separator creates the following
problems4:
a.
Mechanical control of the liquid level is difficult in
the presence of foam, as any control device has to
deal with three phases instead of two.
b. Foam has a large volume and will occupy much of
the vessel space, which would otherwise be available
for liquid collecting or gravity settling.
c. In an uncontrolled foam bank, it is impossible to
remove the separated gas or liquid oil from the
separator without entraining some of the material in
either the liquid or gas outlets.
Of the various methods used to control foam, such
as removing the foam stabilizing agent by filtration
and using mechanical devices or employing
new
equipment designs to break foam, the best method to
accomplish foam removal is the use of an antifoam agent.
Chemical control of foam using such an ‘antifoam’ is
very important.
Gas Separation
& Purification
1995
Volume
9 Number
2
81
Foaming
and carry-over
Components
problems:
affecting
H. 1. Shaban
separation
To have efficient and stable operation over a wide range
of conditions, a gas and liquid separator must have the
features described below1*2.
1.
A primary separation section for removing the bulk
of the liquid from the inlet stream containing liquid
and gas, mainly by centrifugal force. It is essential
to remove slugs and large droplets of liquid from
the gas stream to minimize gas turbulence and
re-entrainment of liquid particles. This is usually
accomplished by a change in direction of fluid flow.
Centrifugal force from a tangential inlet on a vertical
vessel quickly removes large volumes of liquid and
allows redistribution of gas velocity. Properly shaped
and positioned deflection plates are usually used in
horizontal and spherical vessels to accomplish this
effect with a minimum of re-entrainment.
2. A secondary separation section for removing a
maximum of smaller liquid droplets is enhanced by
gravity settling. The major separation principle in
this section primarily depends on gas turbulence. The
turbulence factor is often minimized by a suitable
inlet arrangement
and properly designed and
positioned baffles.
3. In order to separate mist not settled out by
centrifugal force or gravity, a mist extractor is placed
in the way of the gas stream. The mist extractor may
be of the vane or coalescing-pack type, or it might
be a hydrocyclone. The usual oil field separator
employs the impingement principle as the primary
mechanism of mist extraction. The tiny liquid
droplets are collected on a surface, where they are
drained away from the gas stream or form large
droplets that can fall back into the primary
separation section.
4. An oil collecting section for receiving and disposing
of collected liquid. The liquid separated from the well
stream and condensed in the separator collects in the
bottom part of the separator. This section should be
arranged so that the separated liquid has a minimum
level of disturbance from the flowing gas stream. The
liquid level is maintained within given limits by a
liquid level control device, to prevent gas from
entering the oil outlet and liquid from rising to the
elements in the separator, and to ensure sufficient
retention time for the gas bubbles to break out of
the liquid.
Foaming
problems
Foam can seriously impair the operation of petroleum
processes, but it is not often recognized as the major
cause of a particular problem. Many problems, such as
reduced capacity of equipment, large overhead losses,
overhead fouling or poor separation efficiency, may
occur because of foam. Foam occurs when a surfactant,
which is either added intentionally or is naturally present
during a process, dissolves in a liquid. For example, crude
oil may not foam, but once a corrosion inhibitor is added
82
Gas Separation
& Purification
it may foam to such an extent that the crude oil cannot
be processed effectively without the aid of an antifoam’.
Foam
control
One way of controlling foam is to prevent it from
forming
in the first place. This can either be
accomplished by removing the foam stabilizing agent by
either changing or discontinuing the use of a problem
chemical. A simple filtration may selectively remove the
problem chemical and reduce or eliminate the foaming
problem. This approach is not always a possible
alternative.
Surface whips, thermal shock and ultrasonic techniques are various mechanical devices that have been
proposed to control foam. New equipment such as a
cyclone-style gas/oil separator may be used to minimize
the amount of foam. Using the above methods, a
reduction in foaming tendency is usually accomplished
and the use of antifoam is still required.
Chemical control of foam by the use of antifoams is
very important
but is often not well understood.
Distinction is made between antifoams (chemicals added
before the foam has formed to inhibit foam formation)
and defoamers (chemicals added after foam has formed
to knock down the foam). An antifoam is usually a good
defoamer; it functions in the bulk of the liquid to control
foam as the bubbles are nucleating, but will also act on
the bubble surface to destroy the foam. Many organic
materials are good defoamers but poor antifoams’.
Foam depressants will often increase the capacity of a
given separator. However, in sizing a separator to handle
a particular crude, use of an effective depressant should
not be assumed, because the crude and foam
characteristics may change during the life of any field.
Sufficient capacity should be provided in the separator
to handle anticipated production without the use of a
foam depressant or inhibitor. Once in operation, use of
a foam depressant may allow more throughput than the
designed capacity’.
Discussion
carry-over
of poor
problem
separation
and
The carry-over problem has been observed in separators
due to foaming. Such foaming has resulted in the
carry-over of crude into the gas stream and, in addition,
the separators have to operate at their full design
capacity to meet the production target rate set for the
gathering centre. To reduce foaming and the carry-over
problem, the oil level in the separators is lowered and
the total span of the level controllers set at 10%. By
doing this, the oil and gas separation becomes inefficient
in the first stage and causes slippage of gas to the second
stage, exceeding its design capacity and thus resulting in
poor performance. Furthermore, this operating condition results in an increased amount of gas reaching the
high-pressure (HP) flare, as the low-pressure (LP) gas
compressor at the gas booster station is already
operating at full design capacity and cannot process any
additional LP gas. Moreover, the scrubbers in the LP
1995 Volume 9 Number 2
Foaming
CHEMICAL
TANK
and carry-over
problems:
H. 1. Shaban
li-
CHEhllCAL
PUMP
PRV
HP HEADER
1 ~‘1, ,,.n.,r
ALAnM
q AnHt
HP GAS TO
BOOSTER
STATION
\I
FLOW LINES
(HP WELLS)
I
LPHEADER
CARRY
OVER
Al ARM q ADDCL
v.-.....
“,,I,,,L,
1
LP GAS TO
BOOSTER
I
STATION
LP
SEPARATOR
FLOW LINES
(LP WELLS )
CRUDE OIL
TANK
Figure
1
Process
flow
diagram
and HP gas systems at the gas booster station have to
be drained out many times a day due to the excessive
carry-over of crude oil in the gas stream.
Process
description
The dry crude oil from the wells flows through valves
and headers into separators and then to oil tanks, as
shown in Figure 1. The separators and tanks degas the
crude oil. The separators in this particular oil field are
of a horizontal type and are equipped with foam breakers
to improve performance. The mechanical foam breakers
are divided into two parts (see Figure 2). First, the inlet
performs degassing by gently agitating the feed fluid. This
will assist in removing gas from the oil and breaking
foam bubbles as they flow through the inlet element.
2
/4
for
HP and
LP crude
oil separation
The second mechanical foam breaker is defoaming
plates, which extend from near the inlet to the outlet of
the separator. The plates that are immersed in the oil
assist in removing non-solution gas from the oil and in
breaking foam in the oil. The plates that are above the
oil/gas interface in the gas section of the separator
remove oil mist from the gas and assist in breaking foam
that may exist in the gas section of the vesse17.
Excessive foaming was observed in day-to-day
operations and it became evident that this mechanical
means of breaking down foam is not efficient and an
alternative method is therefore required. Table 1 gives
the design specifications of the HP and LP separators.
Chemical
control
of foaming
Silicone fluids have been used for many years to suppress
foaming of crude in separators and to prevent carry-over
of liquid with the gas. To increase the capacity, a silicone
antifoam injection was introduced into the separators in
the unit under study. A silicone injection installation
provided on site was connected to the HP separator inlet
to ensure that the chemical was adequately mixed, as
6
1
Table
1 HP and
LP separator
operating
Description
(1)
(2)
(3)
(4)
(5)
(6)
Well fluid inlet
Inlet degassing element
Defoaming plates
Gas outlet
Oil outlet
Oil level control
Dimensions
(internal
Dimensions
(length)
Flow rate (BPD)
Pressure
(psig)
Temperature
(‘F)
Retention
Figure
2
Horizontal
diameter)
(m)
(m)
(min)
design
specifications
HP
LP
2.5
13
50 000
470
190
4.5
2.5
13
50 000
85
180
4.5
I
separator
Gas Separation
& Purification
1995
Volume
9 Number
2
83
Foaming
and carry-over
problems:
H. 1. Shaban
shown in Figure I. The antifoam is continuously injected
into the feed and stays in the separated oil through the
next processing stages.
The ability of the antifoam to enter the liquid/gas
interface and spread within a developing foam system is
determined by a combination of surface and interfacial
tension. The silicone, with its low surface tension and
low interfacial tension with oils (2.8 to 4.8 dyne cm ‘) is
specially suited to enter and spread in non-aqueous
systems. Polydimethyl siloxane (PDMS) is non-polar,
non-ionic, insoluble in water and hydrophobic in nature.
Silicone fluid against water has a relatively high
interfacial tension and therefore has a poor or reduced
spreading tendency. However, if compounded with silica,
it makes an efficient antifoam for aqueous foams by
entering the film and bursting the bubbles using the
hydrophobic silica via a dewetting mechanism5.
A silicone antifoam may also disrupt the other foam
stabilizing mechanisms. Surface viscosity will be reduced
if the foam stabilizing surfactant is displaced by a silicone
that does not exhibit hydrogen bonding. Displacement
of ionic surfactants by non-ionic silicone destroys
electrical double-layer effects. Similarly, displacement of
the surfactant by gas-permeable silicone will remove the
resistance to gas diffusion between bubbles and allow
this bubble-breaking mechanism to operate.
Table
4 Oil flow rate
amounts
of antifoam
and
(GPD)
LCV
Antifoam
injection
Table
open
Table
0
5
10
15
20
25
30
68
71
78
80
81
81
81
55
57
60
67
69
70
68
injection
on foam
Foam collapsing
time (min)
(GPD)
0
5
10
15
20
Gas Separation
7
4.2
2
1.3
0.9
0.9
1
& Purification
1995
Volume
Oil flow rate
HP and LP
( x 1000 BPD)
50
52.4
54.8
57
57.6
57.7
57.7
5
Effect
of antifoam
on gas
production
Gas production
rate
( x 1000 MSCFD)
Antifoam
injection
(GPD)
HP
LP
0
5
10
15
20
25
30
16
23
29
30.5
35
37
34
12
11
9
5.5
5
5.1
5
6 Effect
Antifoam
injection
of antifoam
on oil level
Oil level
separators
in separators
in
(%)
and throughput
Separator
throughput
(GPD)
HP
LP
0
23.7
20
20
8
13.9
15
15.2
10.2
12.6
15
17.8
Figures
Antifoam
injection
84
(%)
LP
of antifoam
varying
(BPD)
52
52
55
63
724
724
400
000
achieved without carrying-over. All the results are
summarized in Tables 24 and the data are plotted in
HP
Effect
with
prior
(GPD)
3
LP separators
0
5
10
15
20
25
30
Results for the HP and LP systems described previously
are discussed as a function of antifoam addition and its
effects on separation characteristics. The feed was
initially set at the design rate of 50000 BPD for each
stage (HP and LP) and gradually increased to the
maximum
possible throughput
rate that could be
valves
days
and
through
separators
discussion
Table
2 Status
of HP and LP separator
level control
to and after antifoam
injection
over a period
of seven
HP
Antifoam
injection
Table
Results
through
3-5.
As shown in Figure 3, the collapse time curve consists
of two regions. The first part is linear, where collapse
time decreases with increase in antifoam injection rate.
The second region has parabolic behaviour, with a
levelling trend at high antifoam rates. This behaviour is
expected, since, at low antifoam concentrations, the
collapse rate can be assumed to be first order with respect
to antifoam concentration. Similarly, at higher antifoam
concentrations, the rate becomes zero order with respect
to antifoam concentration.
In Figure
4, the curve shows clearly that the
oil flow rate has two regions. The first region is linear,
where the oil production rate increases because foam
concentration decreases with the addition of antifoam.
The second region is parabolic in nature with a levelling
trend at high antifoam injection rates. Also, it can be
seen that there is an optimum region for foam addition,
where levelling of oil flow rate occurs.
Figure
5 shows the effect of antifoam
on gas
production. It shows that the antifoam enhances gas
9 Number
2
Foaming
and carry-over
problems:
H. 1. Shaban
HP
LP
2
0
IS
20
Anli-foam
Figure
0
3
Effect
of antifoam
10
Anti-foam
Figure
varying
30
injection
20
injection
4
Plot for oil flow rate through
amounts
of antifoam
40
0
10
Anti-loam
( GPO )
injection
on foam
Figure
30
(GPO)
HP and
LP separators
with
5
Effect
20
inisction
of antifoam
30
(GPD)
on gas
40
production
release in the first stage (HP) separator, accompanied by
a decrease in gas production in the second stage (LP)
separator, to conserve the total gas mass. Due to this,
the flaring quality also improves in both the HP and LP
flares.
Table 6 shows that adding the antifoam agent while
keeping the throughput constant results in an increasing
oil level in the separators, as shown in the first two runs
in Table 6. Also, keeping the antifoam injection rate high
(20 gpm) enables the operator to increase the throughput
flow rate and, consequently, increases the oil level in the
separators, as shown in the last two runs in Table 6.
The test results in this study indicate that there is a
marked difference in functioning in terms of separator
throughput when silicone antifoam is injected into the
crude stream. The handling capacity of the separators
increased far in excess of their rated nominal capacity.
Operating conditions and separation efficiency were
improved tremendously. Flaring quality improved in the
HP and LP flares and the light ends were retained in the
crude stream. There was no carry-over in the gas to the
booster station. Moreover, the results indicate that the
increased throughput did not cause an inconvenience in
terms of plant operation, as all level control valves
(LCVs) and pressure control valves (PCVs) were well
within their designed operating flow range, and no
abnormal vibration was detected during the test period.
The maximum capacity attained before silicone injection
was 50000 BPD with the operating level set as low as
10% of the total control span of the level controllers.
Gas Separation
& Purification
1995
Volume
9 Number
2
85
Foaming
and carry-over
problems:
H. I. Shaban
Above this level, excessive carry-over occurs due to
foaming. After silicone injection, the feed capacity could
be increased to a maximum of 57 500 BPD and the
separator level could be increased to 15% of the total
control span.
remarkably, reducing wastage, and the available LP gas
was processed at the LP compressor within the design
limit. Efficient separation permitted processing of extra
crude. The processing capacity increased to 57 700 BPD
using the silicone antifoam.
References
Conclusions
Even though various mechanical devices have been
proposed to control foam formation, which is one of
the most significant factors affecting oil and gas
separation efficiency, the chemical control of foam
by the use of antifoam is very important.
Using
silicone, the operating level increased to W 15% of
the total span of the level controllers. Efficient oil/gas
separation was achieved with the antifoam injection set
at optimum
dosage; more HP gas was separated/produced in the first stage separator and processed
at the HP compressor at the booster station.
Simultaneously,
LP gas production
was decreased
86
Gas Separation
& Purification
1995
Volume
9 Number
Chilingarian,
G.V.,
Robertson,
J.O.
and Kumar,
S. Surface
operations
in petroleum
production,
in Decelopmrnts
in Petroleum
Science, Vol 19A, Elsevier, New York, USA (1987), Ch 3, 59-69
Szilas,
A.P. Production
and transport
of oil and gas, in
Developments
in Petroleum
Science, Vol 18B, Part B (Gathering
and
Transport),
Elsevier, New York, USA (1986), 68-69
Amoid,
K. and Stewart,
M. Designing
oil and gas production
systems: How to size and select two phase separators
J World Oil
(NovjDec
1984)
Worely, MS. and Laurence,
L.L. Oil and gas separation
is a science
J Petrol Technol (April 1957)
Callaghan,
I.C., Gould, C.M., Reid, A.J. and Seaton, H.D. Crude
oil foaming problems
at the Sullom Voe terminal
J Petrol Technol
(Dee 1985)
Engineering
Data Book 10th Edn, Vol 1, Section
1. Gas Processors
Suppliers
Association
(1987)
Bradely,
H.B. Petroleum
Engineering
Hand Book, Richardson,
USA,
Ch 12, I-40
2